The direct photolysis and photocatalytic degradation of alachlor at different TiO2 and UV sources

Direct photolysis and photocatalytic degradations of alachlor, a widely used herbicide, were studied using three different monochromatic UV lamps (254, 300 and 350 nm) and two TiO2 sources. Both the direct photolysis and photocatalytic degradations of alachlor follow pseudo-first-order decay kinetics. TiO2-P25 was found to be an effective photocatalyst compared to TiO2-BDH. The direct photolysis of alachlor was dominant at 254 nm even if TiO2 was present in the solution. Among the three UV wavelengths used, the highest photocatalysis quantum yield was obtained at 300 nm. The photocatalytic degradation rate of alachlor increased with the dosages of TiO2, but an overdose of TiO2 would retard the reaction due to light attenuation. Photocatalytic reactions were slightly enhanced in an alkaline medium, and the different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. This effect was diminished at a later stage after the reaction intermediates were formed.